1. Field of the Invention
The present invention relates to a transmission power control circuit which is used, for example, for the final stage or driver in a mobile communication transceiver.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a transmission power control circuit used as a transmission stage of a mobile communication transceiver as known in the prior art. In FIG. 1, numeral 1 designates an RF power module which amplifies and outputs transmission signal TX, numeral 2 designates a detection circuit which detects the output of the RF power module 1, and numeral 3 designates a multiplexer which selects, for example, one of six level-setting steps in 4 dB per step in response to a control signal 3a.
Numeral 4a designates a DC amplifier which amplifies output signals of the detection circuit 2, numeral 4b designates a DC amplifier which amplifies output signals of the multiplexer 3, numeral 4c designates a DC amplifier which supplies the multiplexer 3 with a reference input voltage, numeral 5 designates a current controlled differential amplifier which is composed of transistors 5a, 5b fed the outputs of the DC amplifiers 4a, 4b and numeral 6 designates a power control transistor which inputs a power source bias voltage (+6 V, for example) from a terminal 7 in accordance with output of the current control differential amplifier 5 and outputs a voltage of prescribed level to the RF power module 1.
Numeral 10 designates a terminal for transmission of an ON/OFF signal applied to the bases of transistors 11a, 11b. The emitter of each of transistors 11a, 11b is connected to a negative power source of -5 V, for example, through a terminal 13.
FIG. 2 is a circuit diagram showing an example of circuit constitution of the RF power module 1 using FETs. Also FIG. 3 is a circuit diagram of the multiplexer 13 showing the inner constitution of the C-MOS IC having a multiplexer function. The C-MOS IC is provided with three control signal terminals, three output terminals and six input terminals, but three of the six input terminals are only used in this case. In operation each input signal at input terminals corresponding to control signals 3a is switched so as to output the respective output terminals.
The operation of the transmission control circuit of FIGS. 1 to 3 will now be described. Transmission signal TX is inputted to the RF power module 1 and amplified and outputted as transmission output (TX OUT). (TX OUT) is also detected with respect to high frequency signal portions by the detection circuit 2, and is further rectified, smoothed, and transmitted to the DC amplifier 4a as a DC signal corresponding to the output of RF power module 1.
On the other hand, transmission ON/OFF signal from terminal 10 is applied to the bases of the transistors 11a, 11b, and the transistor 11a is turned on by ON-signal of the transmission ON/OFF signal from terminal 10 and the emitters of the transistors 5a, 5b become the potential of -5 V thereby the current control differential amplifier 5 is biased into an active state.
The multiplexer 3 selects a reference input voltage in by the combination applied to the three input terminals corresponding to one of six steps of the encoded control signals 3a, and sends outputs to the three output terminals. These combined outputs are converted together via resistors having different resistance values connected to the output terminals of the multiplexer 3 into an output control signal for one of six level steps, and supplied to an inverting input terminal of the DC amplifier 4b. The output control signal is amplified by the DC amplifier 4b and becomes one input of the differential amplifier 5. Outputs of both DC amplifiers 4a, 4b are amplified in differential by the current control differential amplifier 5, and applied to the base of the power control transistor 6.
Thereby the power control transistor 6 controls the voltage supplied to the RF power module 1, and performs stabilization of the transmission power and the transmission power control of six steps in 4 dB per step up to the maximum transmission power.
As shown in FIG. 2, in the RF power module 1 using FETs, voltage applied to drain voltage terminals D1, D2 is controlled in six steps corresponding to the output of the differential amplifier 5.
FIG. 4 is an operating characteristic diagram showing the I.sub.D -V.sub.GS characteristics of the FET. As shown in FIG. 4, if the operating point is set to the pinch-off voltage V.sub.P, prescribed output cannot be obtained. Consequently, in order to obtain the prescribed output without using an FET of large output, the FET is usually used in the state that the operating point is set higher than the pinch-off voltage V.sub.P. In this case, however, even if voltage is not applied to gate, drain current flows as idle current. When the power control is performed, ratio of the idle current versus drain current corresponding to the output power becomes large thereby efficiency of the RF power module 1 becomes low. That is, even if the efficiency is optimum at a certain step of the stepwise level control, it may be lowered significantly at other steps.